Question: Solve for $z$ : $12 = 20 + z$
Explanation: Subtract $20$ from both sides: $ \begin{eqnarray} \\ \hphantom{12 {- 20}} &\hphantom{=}& \hphantom{z} \\ 12 &=& 20 + z \\ \\ {-20} && {-20} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 12 {- 20} &=& z \\ \end{eqnarray} $ Simplify. $ z = -8$